Legacy Private Branch Exchange (PBX) systems allow a number of users to share a smaller set of telephone lines that are connected to a public telephone network (PSTN). Since it has been more cost effective to purchase a PBX switch and deploy it within a facility than pay monthly access fees for connecting every telephone to a switch in a public network, usage of legacy PBXs is currently widespread.
With the advent of Session Initiation Protocol (SIP), a powerful signaling protocol for creating, modifying, and terminating multimedia sessions, including Internet telephone calls between one or more SIP-endpoints, many legacy PBX users want to migrate portions of the SIP technology into their existing infrastructure so that they can (1) retain all of their existing functionality, (2) take advantage of the many services SIP provides and (3) avoid the cost of purchasing a new IP-PBX system and its associated equipment (e.g., user sets) that already (or can as an option) internally incorporates the SIP technology. Details about the SIP signaling protocol are set forth in Internet Engineering Task Force Request for Comment 2543 entitled “SIP: Session Initiation Protocol,” March 1999 (hereinafter referred to as RFC 2543), which is incorporated herein by reference, SIP provides an alternative to PBX- or H.323-signaled telephony.
Accordingly, there is a need for a hybrid system that externally integrates a PBX system with a SIP network.